Commercially there is a large demand for a stable and economical laser source in the blue to green range including the UV to visible spectrum, for instance for high density optical storage retrieval and biomedical applications. Laser output in this frequency range is provided by coupling a higher frequency semiconductor laser to a non-linear crystal harmonic frequency converter. An example is SHG (second harmonic generation) which generates a second harmonic beam, where the frequency of the second harmonic beam is twice the frequency of the pump beam.
These harmonic frequency converters are highly sensitive to polarization and wavelength. Accordingly, alignment and temperature stability are critical to reliable operation.
Providing an optical fiber coupling between the semiconductor laser and the harmonic frequency converter greatly simplifies alignment issues in assembly. In addition the fiber laser has a well controlled line width including a fiber Bragg grating (FBG) to lock the pump laser at the FBG wavelength. The cavity is formed between the back side of the chip which has an HR (high reflectivity) coating or close to HR (a 95% reflectivity coating is provided in a preferred embodiment, using 5% for photodiode monitoring in the back) and the grating, allowing many modes under the spectral envelope of the grating. The longer the cavity the shorter the mode spacing. Many modes facilitate RMS (root mean square) noise reduction involving mode competition averaging. Therefore, the use of a fiber laser is a simple way to achieve a longer cavity with the wavelength selectivity required.
The optical link between the laser diode and a harmonic frequency converter is a singlemode optical fiber, comprising the laser cavity, the narrow band grating and the optical link to the harmonic frequency converter. Experimentation with a PM fiber link to maintain a polarized output from the laser transmitted to the polarization sensitive converter revealed that this structure does not provide a consistently stable output amplitude.
U.S. Pat. No. 5,966,391 discloses the use of rare earth doped PM fiber for providing gain while simultaneously coupling a single linear polarization into the nonlinear crystal of a harmonic frequency doubler.
U.S. Pat. No. 6,683,902 discloses the use of a 90 degree splice of PM fiber within the cavity of an semiconductor external cavity laser in order to reduce the phase difference within the cavity to reduce the transverse mode competition and stabilize the power of the laser.
It is recognized that PM fiber is temperature sensitive, with changes in temperature causing the birefringence, the Δn difference in refractive index between the orthogonal axes, to vary.
In optical fiber sensors, it is recognized that the variation in birefringence of PM fiber can be temperature compensated by splicing two lengths of polarization maintaining fiber at 90 degrees with respect to the fast axis of each, such that the effects of temperature do not cause additional phase shift between light launched into the fast and the slow axes. For example, U.S. Pat. No. 4,773,753 issued Sep. 27, 1988 to Takao Hirose et al. discloses a fiber sensor, including a 90 degree splice of birefringent fibers selected by material or length to eliminate temperature dependent phase change, for measuring temperature or strain.
However, none of the prior art recognizes the problems causing power fluctuations in the output of a fiber laser and harmonic frequency conversion module, nor suggests a structure or method to provide a fiber laser harmonic frequency conversion device having a stable output.
Accordingly, a fiber laser and harmonic frequency conversion module, having a stable output power remains highly desirable. It is a further object to provide a fiber laser harmonic frequency conversion module having reduced temperature sensitivity.